This invention relates generally to methods and apparatus for detecting and controlling the accumulation or buildup of a substance on a selected surface, and more particularly relates to methods and apparatus for detecting and controlling the buildup or ice or frost on a refrigerated surface. In addition, the invention relates to processes and apparatus for preventing frost formation on a refrigerated surface.
In modern refrigerating equipment, the accumulation of ice or frost on the evaporator coils represents a problem. The accumulation or buildup of ice or frost insulates the air to be cooled or refrigerated from the cold refrigerating fluid circulating through the evaporator unit, resulting in a difficulty in maintaining the refrigerated space at the desired temperature.
The prior art has attempted several techniques to solve this problem. One conventional method widely used provides for a time clock which at some preselected time interval shuts off the compressor and turns on a thermal heating device adjacent to the evaporator coils to melt the ice or frost. The disadvantage of this technique is that ice or frost does not always accumulate at a constant rate, depending on the ambient humidity and temperature of the air, and most clock-actuated defrost systems defrost oftener than necessary, out of an abundance of caution, in order to eliminate all buildup of ice or frost. This defrosting on a regular time cycle, whether needed or not, independent of the quantity of accumulated ice or frost, is inefficient and wastes electrical power. Another method is simply to shut off the compressor for a predetermined length of time generally sufficient to allow the ice or frost to melt. However, this is also inefficient, since the turning off of the compressor permits the refrigerated space temperature to rise and again make the compressor and its motor work excessively to maintain the desired temperature. Other techniques use temperature sensitive devices to measure the temperature of the refrigerating medium and signal the defrost cycle upon a predetermined rise in the temperature as detected by the device. These temperature sensitive devices must be extremely sensitive to detect a small change in the temperature of the refrigerating medium which corresponds to a wide variation in the thickness of the accumulation of the ice or frost and thus are generally not very accurate.
Other methods of detection include optical means to detect buildup or accumulation of ice or frost. One such technique utilizes a light source and a photocell oriented to transmit and receive the light adjacent the evaporator coils or the fins associated with such coils. As the ice or frost accumulates, it physically blocks the passage of light to the photocell and can trigger appropriate ice or frost controlling apparatus. Such prior art is disclosed in the following U.S. Pat. Nos.: 2,297,370; 2,355,014; 2,377,926; 2,446,885; 3,120,108 and 3,188,828. None of these patents discloses the transmission of a beam of electromagnetic radiation of a preferred or selected wavelength and selected intensity through the ice or frost for measuring the change in intensity due to a change in the thickness of the ice or frost and resulting from absorption or scattering of the radiation by the frost. Of particular interest is the last enumerated U.S. Pat. No. 3,188,828, issued to Wayne on June 15, 1965. Wayne discloses apparatus for detecting ice utilizing a light source and photocell detector. The light source directs light upon a selected surface. The photocell is spaced from the light source and shielded from the light source. As ice forms on the selected surface, the light from the source reflects, scatters and refracts the light within the frost or ice, and a portion of such reflected, scattered and refracted light reaches the photocell and generates a signal in response thereto. In theory, as the ice builds up the amount of light reaching the photocell increases, and the signal produced by the photocell will increase in direct proportion to the increased thickness of the ice. This method depends on the transmission and scattering around a barrier. Since this method depends on the detection of photons which have traveled within the ice or frost and around a barrier and which eventually exit the frost at a secondary site, the change in intensity to be detected is small compared to the ambient background. Accordingly, the light measuring technique disclosed in U.S. Pat. No. 3,188,828 can be accurate only within rather narrow limits of application and is not suited to widespread use under varying applications of all types of refrigerating equipment, as is the case of the present invention.
Of interest also is U.S. Pat. No. 3,280,577 to Kobayashi, which discloses the use of a detector and a visible light source for directing the visible light through a layer of frost to the detector for producing electrical signals and means for controlling a defrosting operation in response to such electrical signals. The changes in the visible light passing through the ice or frost are caused by virtue of scattering and absorption of the visible light by the frost. However, the sensitivity of such a control disclosed in Kobayashi will be low since it is attempting to utilize a broad spectrum of visible light as a source and attempting to detect such visible light over a broad range, the limitations of which will be hereinafter further treated. Further, use of a high intensity incandescent light limits structural miniaturization and produces unwanted quantities of heat.
Accordingly, one primary feature of the present invention is to provide an ice or frost detecting apparatus of great accuracy, high reliability and low cost.
Another feature of the present invention is to provide an ice or frost detecting apparatus that can have universal application in all types of refrigerating equipment.
Yet another feature of the present invention is to provide an ice or frost detector that can accurately determine any thickness of ice or frost accumulated on the surface of evaporating coils of refrigerating equipment.
Still another feature of the present invention is to provide a control circuit, responsive to the ice or frost detector, that will control the buildup of ice or frost on the evaporator coils of refrigerating equipment.
Another feature of the present invention is to provide means responsive to a control circuit for melting ice or frost on evaporator coils that allows for continued operation of the refrigerating equipment during the defrost cycle.
Yet another feature of the present invention is to provide mcans for inhibiting the formation of ice or frost on selected evaporator coil surfaces of refrigerating equipment.
Still another feature of the present invention is to provide means for detecting the level of a liquid.
Another feature of the present invention is to provide means for detecting the physical change in state of a substance.
Yet another feature is to provide a means for the prevention of the condensation of water vapor on evaporator coils and the subsequent accumulation of frost or ice on a refrigerated surface.
Another feature is to provide a means for ice and frost control which has increased sensitivity over prior art devices.